PHP Manual :: Expiration Times

Some storage commands involve sending an expiration value (relative to an
item or to an operation requested by the client) to the server. In all such
cases, the actual value sent may either be Unix time (number of seconds
since January 1, 1970, as an integer), or a number of seconds starting from
current time. In the latter case, this number of seconds may not exceed
60*60*24*30 (number of seconds in 30 days); if the expiration value is
larger than that, the server will consider it to be real Unix time value
rather than an offset from current time.

If the expiration value is 0 (the default), the item
never expires (although it may be deleted from the server to make place for
other items).